1. Field of the Invention
The present invention related to closed-loop fiber optic gyros. More particularly this invention pertains to a method for compensation for temperature-dependent drift changes in a fiber optic gyro, and to a fiber optic gyro that operates according to that method.
2. Description of the Prior Art
In fiber optic gyros, two light beams that originate from a single light source are injected in opposite directions into a fiber coil. Once they have passed through the coil in opposite directions, they are recombined to produce an interference pattern on a detector. If the coil is rotated about its axis, the Sagnac effect generates a non-reciprocal phase shift between the two light beams that produces a shift in the interference pattern. The intensity and direction of the interference shift is proportional to ration direction and rate of the coil about its axis.
When a fiber optic gyro is reset, the detector output signal is processed by electronic control loops to form a non-reciprocal phase shift that is applied to a phase modulator (generally located at one end of the fiber coil) to compensate for the Sagnac phase shift (produced by rotation) between the two counterpropagating light waves. In such fiber optic gyros, the non-reciprocal phase shift to compensate for the interference shift produced by rotation of the coil is dependent upon rotation W in accordance with the following equation: EQU .phi.=S.multidot.W,
where S defines the scale factor. The scale factor S is dependent upon changes in the length of the coil fiber. EP 0 245 118 B1 teaches stabilization of scale factor S by measurement and correction of the fiber length in a gyro. To do this, means are provided for comparing the detector output signal with a signal derived from the phase modulation. This results in the production of a signal that is proportional to the current optical length of the fiber coil. Such signal is employed to control the frequency f.sub.M of the phase modulator such that:
f.sub.M =1/2.tau.=c/2nL
In this case, .tau. is the time for light to propagate through the coil, c is the speed of light, n is the refractive index of the fiber and L is the length of the fiber on the coil. The term f.sub.M is thus a measure of the optical fiber length of the coil. If the refractive index n is constant, the regulated modulation frequency can be employed (as is known) for scale factor correction to avoid sudden major intensity changes at the detector. If, on the other hand, the refractive index n changes, such scale factor correction is unusable as f.sub.M is dependent on it, but the scale factor S is not.
The above-referenced patent document also fails to address the fact that changes in temperature cause drift changes over time in the fiber gyro that differ for heating and cooling. This behavior, which is known in principle as the Shupe effect, depends indirectly upon the temperature and its time derivatives and--as more detailed investigations show--is influenced by:
the coefficients of expansion of the cladding and core material of the optical fiber, the coating, the adhesive layers and the coil former; PA1 the variation of refractive index with temperature; and PA1 the change in the internal pressure in the coil, produced by the materials, mentioned above, being of different coefficients of expansion, which also acts on the refractive index and on the increase in length.
The Shupe effect can be reduced by so-called quadrupole coil winding. With regard to residual drift, which can still be observed, multiple influencing parameters imply that a temperature model dependent upon the temperature T and its time derivatives T' and T", is complex and nonlinear. Repeatability and long-term stability are, therefore, particularly poor since internal pressure differences can be equalized in the long term, particularly in the organic materials employed. Furthermore, temperature sensors cannot be placed in the fiber winding of the coils. Due to the poor thermal conductivity of fiber coils, the termperature measurements generally do not provide a good representation of the coil windings' temperature. Such temperature models for limiting the Shupe effect are known, but offer no satisfactory solutions.